our lib::P smart-pointer is built on top of std::shared_ptr,
while additionally delegating comparisons to the pointee.
In a similar vein, I've now added a custom string conversion,
delegating to the pointee, with a type-string as fallback.
Together with the built-in string conversion for output streams,
we should now be able to remove most of the explicit string
conversions and calls to util::str in all of our test code.
This removes the last roadblock towards disentangling the
pretty-printing header includes, which in turn should allow
us to remove any conditional code in the built-in string
conversion of GenNode, Variant and the like. Which basically
was the objective for ticket #985
